Thermoplastic parisons are heated prior to a blow molding operation by placing them in thermal proximity to a heating device such as, for example, a tungsten heating element housed in a quartz evelope which emits short wave infrared radiation to penetrate the parisons. A significant drawback in using an infrared energy source is that the impinging radiation does not evenly penetrate the wall of the parison. Thus, it became necessary to extend the amount of time that the parison is exposed to the heating element to adequately heat the wall of the parison all the way through to the interior surface of the parison. However, this solution creates several other problems. First, the extended exposure often causes the exterior surface of the parison to overheat. This is unacceptable for many types of materials, such as, for example, polyethylene terephthalate (PET) which crystalizes at elevated temperatures. Second, a relatively high temperature differential or gradient is established across the wall of the parison and is an undesirable condition for the blow molding operation. Third, the cycle time required for the entire blow molding operation is increased. Several attempts have been made to overcome these problems. One such method includes simultaneously heating and cooling a parison as disclosed in U.S. Pat. No. 4,076,071 granted on Feb. 28, 1978, to O. Rosenkranz, et al. The parison is heated while passing adjacent an infrared heating device and simultaneously cooled by coolant forced between the heater and the parison. Another method disclosed in U.S. Pat. No. 4,079,104 granted on Mar. 14, 1978 to A. C. Dickson, et al., includes simultaneous heating and cooling wherein a coolant impinges directly on the heating elements and then on the parison surface. Both of these solutions have introduced new problems inherent in apparatus for simultaneously heating and cooling. Not only is such apparatus more costly, but also more difficult to control and maintain.